Compounds of formula (I), and pharmaceutically acceptable salts thereof
wherein
R₁ is hydroxy, amino, chloro or OR₇
wherein
R₇ is C₁₋₆ alkyl, phenyl or phenyl C₁₋₂ alkyl either of which phenyl moieties may be substituted by one or two substituents selected from halo, C₁₋₄ alkyl or C₁₋₄ alkoxy;
R₂ is amino or, when R₁ is hydroxy or amino, R₂ may also be hydrogen;
R₃ is hydrogen, hydroxymethyl or acyloxymethyl;
R₄ is a group of formula:
R₅ and R₆ are independently selected from hydrogen, C₁₋₆ alkyl and optionally substituted phenyl; or
R₃ and R₄ together are:
wherein
R₆ is as defined above;
having antiviral activity, to processes for their preparation and their use as pharmaceuticals.